Supermaneuverability: The X-31 Program

NASA Langley became involved in the X-31 Enhanced Fighter Maneuverability (EFM) program in 1984, when mutual discussions with Rockwell International occurred regarding a fighter configuration capable of highly agile flight at extreme angles of attack. Known as the Super Normal Attitude Kinetic Enhancement (SNAKE) configuration, the design underwent exploratory testing in the Full-Scale Tunnel.[1319] The
early cooperative research study later led to a cooperative project using the Langley Full Scale Tunnel, the Langley Spin Tunnel, and the Langley Jet-Exit Test Facility. After DARPA and the West Germany government formally initiated the X-31 program, Langley and Dryden actively par­ticipated in the development of the configuration and flight tests of two X-31 demonstrators at Dryden from 1992 to 1995.

In the early SNAKE Langley-Rockwell study, Langley researchers assessed the high-angle-of-attack capabilities of the Rockwell-designed configuration that had been designed using computational methods with minimal use of wind tunnel tests. Preliminary evaluations in the full-scale tunnel disclosed that the configuration was unacceptable, being unsta­ble in pitch, roll, and yaw. Langley’s expertise in high-angle-of-attack stability and control contributed to modifications and revisions of the original configuration, eliminating the deficiencies of the SNAKE design.

Simultaneous with the SNAKE activities, several other events con­tributed to shaping what would become the X-31 program. First, the emerging recognition that thrust vectoring would provide unprece­dented levels of control for precision maneuvering at extreme angles of attack had led to joint Langley-Rockwell studies of jet-exit vanes sim­ilar to those previously discussed for the Navy F-14 experiments and the NASA F/A-18 HARV vehicle. The tests, which were conducted in the Langley Jet-Exit Test Facility, inspired Rockwell to include multi­axis thrust-vectoring paddles in the SNAKE configuration. Free-flight testing of the revised SNAKE configuration provided impressive proof that the vectoring paddles were extremely effective.

The second major activity was the strong advocacy of the West German Messerschmitt-Bolkow-Blohm (MBB) Company that asserted that high levels of agility for poststall flight conditions provided dom­inant capabilities for close-in air combat. With the support of DARPA, the X-31 EFM program was initiated in 1986 with a request that Langley be a major participant in the development program. Using the NASA Langley test facility assets for free-flight model testing, spin testing, and drop-model testing uncovered several critical issues for the configuration.

One issue was the general character of inherent poststall motions that might be encountered in the aircraft flight program. Results indi­cated that the X-31 might have marginal nose-down control for recovery from high-angle-of-attack maneuvers, and that severe unstable wing – rock motions would be exhibited by the configuration, resulting in a violent, disorienting roll departure and an unrecoverable inverted stall

condition. With these inputs, the X-31 design team worked to configure the flight control system for maximum effectiveness and to prevent the foregoing problems, even without thrust vectoring. The value of these contributions from Langley cannot be understated, but equally impor­tant contributions were to come as the drop-model technique maintained operations during the full-scale aircraft flight-test program.

Flight-testing of the two X-31 aircraft began at Dryden in February 1992 under the direction of an International Test Organization (ITO) that included NASA, the U. S. Navy, the U. S. Air Force, Rockwell, the Federal Republic of Germany, and Deutsche Aerospace (formerly MBB). Two issues were encountered in the flight-test program, resulting in addi­tional test requirements from the supporting team of Langley research­ers. Early in the flight tests, pilots reported marginal nose-down pitch control and said that significant improvements would be necessary if the aircraft were to be considered an efficient weapon system for close – in combat. In a quick-response mode, Langley conducted evaluations of 16 configuration modifications to improve nose-down control in the Full-Scale Tunnel. From these tests, a decision was made to add strakes to the lower aft fuselage, and pilots of subsequent flight tests with the modified airplane reported that the problem was eliminated.

Another problem encountered in the X-31 flights at extreme angles of attack was the presence of large out-of-trim yawing moments with the potential to overpower corrective inputs from the pilot. After a depar­ture was unexpectedly experienced during a maneuvering flight near an angle of attack of 60 degrees, analysis of the flight records indicated that the departure had been caused by a large asymmetric yawing moment that was much larger than any predicted in subscale wind tunnel testing. The presence of asymmetric moments of this type had been well-known to the aeronautics community, including the fact that the phenomenon might be sensitive to the specific Reynolds number under consider­ation. Experience had shown that, for some configurations, the out-of­trim moments exhibited during subscale model tests might be larger than those exhibited at the full-scale conditions, and for other config­urations, opposite results might occur. In the case of the X-31, the full – scale aircraft exhibited significantly higher values.[1320]

The flight-test team sent an urgent request to Langley for solutions to the problem. Once again, tests in the full-scale tunnel were conducted of a matrix of possible airframe modifications, a candidate solution was identified, and real-time recommendations were made to the ITO. In these tunnel tests, a single nose strake was used to predict the maximum level of asymmetry for the airplane, and the solutions worked for that configuration. A pair of nose strakes designed in the tunnel tests was
implemented and, together with other modifications (grit on the nose boom and slight blunting of the fuselage nose tip), permitted the air­craft flight program to continue. This X-31 experience was noteworthy, in that it demonstrated the need for testing seemingly unimportant details at Reynolds numbers equivalent to flight.

The X-31 EFM program completed an X-plane record of 524 flights with 14 evaluation pilots from the sponsoring organizations.